Vehicle Fuel Economy by Optimizing Effective Rolling Tire Resistance

ABSTRACT

The subject matter of the present invention relates generally to a vehicle that has axles with tires mounted thereon with at least one axle that is a lift axle, and more specifically, to a method that optimizes the effective tire rolling resistance by adjusting the load on the tires, resulting in an improvement in the fuel economy of the vehicle. According to one embodiment, the method takes into consideration the rolling resistance characteristics of the tires placed onto the axles of the vehicle and provides an algorithm for optimizing their rolling resistance by raising or lowering the lift axle.

FIELD OF THE INVENTION

The subject matter of the present invention relates generally to avehicle that has axles with tires mounted thereon with at least one axlethat is a lift axle, and more specifically, to a method that optimizesthe effective tire rolling resistance by adjusting the load on thetires, resulting in an improvement in the fuel economy of the vehicle.

BACKGROUND OF THE INVENTION

Fuel is the largest operating cost for a typical truck fleet, and mucheffort has gone into improving the fuel economy of heavy trucks. Onecommon configuration for a long-haul tractor trailer is a 6×4 tractor,consisting of one steer axle and two drive axles, pulling a trailercomprising two trailer axles. As a fuel-saving measure the 6×2 tractorhas been introduced to the market. The 6×2 tractor consists of a steeraxle, a non-driven “tag” axle, and a single drive axle. This eliminatesone of the differentials from the drivetrain, simplifies the tag axle,reduces mass and drivetrain friction, and significantly improves fueleconomy. However, drive traction is reduced since the number of tirestransmitting the engine torque to the road has been reduced by half.This is usually only an issue at very low speeds in low gear on lowtraction surfaces.

To address this need for greater traction, axle manufacturers haveintroduced a system based on a 6×2 lift axle. A lift axle is capable oftransferring load from the tag axle to the drive axle, increasingtraction on the drive tires. The amount of load on the drive axle can bedetermined, e.g., by sensors. Means known in the art for measuring thisload are described by U.S. Pat. No. 5,193,063, col. 6, lines 25 thru 45and include placing a load cell between the axle and its suspension,placing a strain gauge between the axle and its suspension point, andmeasuring the pressure in the air springs when they are employed using apressure transducer. Using suitable means for measuring the load, thetag axle can be lifted until the drive axle load reaches a predeterminedvalue. The system then returns to normal loading based on variouscriteria. For example, the load may return to normal either when thevehicle speed exceeds a predetermined (low) threshold, a predeterminedtime passes, or the driver manually turns off the system. The drive axlemay be loaded beyond the usual max load of the tires, as permitted bythe T&RA tables of “Load and Pressure Adjustments at Reduced Speeds”.For example, the drive axle may be loaded to 20,000 lbs or 26,000 lbs atvery low speeds for startup traction.

The means used for lifting or lowering the lift axle include hydrauliccomponents, pneumatic components, and mechanical linkages orcombinations thereof. For examples of such systems, see U.S. Pat. Nos.4,854,409; 5,193,063; 5,230,528 and 7,222,867.

By way of further example, FIGS. 1 and 2, and a written descriptionthereof, of U.S. Pat. No. 7,222,867, which is assigned to InternationalTruck Intellectual Property Company, LLC., are reproduced herein. Avehicle 10 is shown in FIG. 1 that is comparable to a 8×4 or 8×2tractor. Vehicle 10 can be any vehicle configured to haul large andvarying loads. Vehicle 10 includes a chassis 12 with front and rearfixed axles 14,16, 18, which in tum have wheels 20 mounted thereon tosupport chassis 12 above a road surface. Chassis 12 carries a bodyincluding a driver cab 22 and a cargo body 24, such as a dump body.Because the load carried by vehicle 10 varies greatly it can beadvantageous to lower a supplementary axle to avoid having the vehicleviolate per axle loading limitations. Here a lift axle 26 is provided assuch a supplementary axle. Those skilled in the art understand that fulltime use of such an axle raises vehicle operating costs due to increasedrolling resistance.

Automatic operation of lift axle 26, or, alternatively, givingindication to an operator of appropriate times to raise or lower liftaxle 26, involves other vehicle systems which are schematicallyillustrated in FIG. 2, which discloses a system for a truck that iscomparable to 6×2 tractor. Chassis 12 is equipped with an air suspensionsystem in which air filled bladders (air springs 44) take over much ifnot all of the support and shock isolation functions of conventionalsolid springs. Among the advantages of air springs is that the quantityof air in them can be adjusted to maintain chassis 12 at a fixed height.To this end an air delivery system works through a height leveling valve52. Air pressure in the air spring 44 is thus correlated with vehicleload. A pressure sensor 322 is provided for each air spring 44 circuitand provides the basic data for the determination of axle load.Typically there will be only one such circuit per vehicle, however,other arrangements are possible, including individual control for eachair spring and intermediate arrangements, such as the two circuit designillustrated in the figure.

Additional suspension stabilizing linkages 66 are associated with eachair spring 44 depending from frame side rails 48. Air lines 62 connectto a compressed air tank 68 installed on chassis 12 between side framerails 48. An engine 70 provides motive power for chassis 12, driving apropeller shaft 76 by an automatic or semi-automatic transmission 72.Propeller shaft 76 is connected between the transmission 72 and adifferential 74 for the single drive axle 16 shown. A tachometer 75 iscoupled to propeller shaft 76 and allows the determination of theaverage rotational velocity of the drive 20 wheels from which vehiclespeed is estimated. Lift axle 26 is not driven. Pneumatic positioningcylinders 64 are mounted between chassis 12 and lift axle 26 to raise orlower the lift axle as required by the electronic control system.

Heavy trucks, such as shown in FIG. 1, spend much of their time carryingless-than-maximum weight loads. See FIG. 3 where the percentage of usagetime is plotted versus vehicle load. This graph indicates that suchtrucks carry less than their maximum allowable load about 60% of thetime they are used. This can be attributed to the nature of their cargo,i.e. certain fleets are “volume limited” rather than “weight limited”(e.g. wood chip haulers). In addition, many trucks lighten their loadprogressively over the course of their route (e.g. gasoline tankers).Alternatively, other trucks such as dump or garbage trucks may increasetheir load progressively.

Accordingly, as the data in FIG. 3 indicates, the majority of trucks onthe highway would benefit from a tire rolling resistance optimizationscheme that takes into account the load placed on tires. Morespecifically, a method for optimizing the fuel economy of a vehicle thattakes into account the rolling resistance characteristics of a tire andappropriately alters the load on the tire would be beneficial. Such amethod that uses equipment already present on the vehicle, such as alift axle, would be particularly advantageous.

SUMMARY OF THE INVENTION

Aspects and advantages of the invention will be set forth in part in thefollowing description, or may be obvious from the description, or may belearned through practice of the invention.

The present invention includes a method for improving the fuel economyof a vehicle by optimizing effective tire rolling resistance, the methodcomprising the following steps:

providing a vehicle with at least one principal axle and one lift axle,said axles having tires mounted thereon; and

determining whether the total load carried by the vehicle is greaterthan the sum of maximum allowable axle load multiplied by the number oflift axles added to the minimum allowable axle load necessary multipliedby the number of principal axles; or

determining whether the total load carried by the vehicle is less thanor equal to the sum of maximum allowable axle load multiplied by thenumber of lift axles added to the minimum allowable axle load necessarymultiplied by the number of principal axles while also being greaterthan the total number of axles multiplied by the minimum allowable axleload necessary, or

determining whether the total load carried by the vehicle is less thanor equal to the total number of axles multiplied by minimum allowableaxle load necessary.

According to which regime above is found, a lift axle is moved to adjustthe loads on the various axles of the vehicle.

The present invention also includes a method for improving the fueleconomy of a vehicle by optimizing effective tire rolling resistance,the method comprising the following steps:

providing a vehicle with at least one principal axle and one lift axle,said axles having tires mounted thereon; and

determining whether the quantity of the total load carried by thevehicle minus the product of the maximum allowable axle load multipliedby the number of principal axles is greater than the minimum allowableaxle load necessary multiplied by the number of lift axles; or

determining whether the quantity of the total load carried by thevehicle minus the product of the maximum allowable axle load multipliedby the number of principal axles is less than or equal to the product ofminimum allowable axle load multiplied by the number of lift axles whileat the same time the total load is greater than or equal to the totalnumber of axles multiplied by the minimum allowable axle load necessary,or

determining whether the quantity of the total load carried by thevehicle is less than the total number of axles multiplied by the minimumallowable axle load necessary, or

determining whether the quantity of the load carried by the vehicleminus the product of the maximum allowable axle load multiplied by thenumber of lift axles is less than or equal to the product of the minimumallowable axle load multiplied by the number of principal axles while atthe same time the total load is greater than or equal to the totalnumber of axles multiplied by the minimum allowable axle load necessary.

According to which regime above is found, a lift axle is moved to adjustthe loads on the various axles of the vehicle.

A system for improving vehicle fuel economy for a vehicle having atleast one lift axle and one principal axle, comprising:

tires mounted on the lift and principal axles;

an input device for entering instructions or information;

a memory for storing instructions, data or programs having algorithms;

load sensors for determining the loads on the principal and lift axles;

a lift mechanism for lowering or raising the lift axle;

at least one processing device in communication with said input device,said memory, said load sensors, and said lift mechanism, said processingdevice configured for

receiving measurements indicative of the load placed on the principaland lift axles as well as the total load carried by the vehicle;

executing an algorithm depending on whether the tires on the lift axlehave rolling resistance characteristics that are higher, lower or thesame as the tires on the principal axle and subsequently sending asignal to the lift mechanism in order to move the lift axle to changethe loads placed on the principal and lift axles;

stopping the movement of the lift mechanism once the desired loads onthe principal and lift axles have been reached; and,

monitoring the total vehicle load over time.

These and other features, aspects and advantages of the presentinvention will become better understood with reference to the followingdescription and appended claims. The accompanying drawings, which areincorporated in and constitute a part of this specification, illustrateembodiments of the invention and, together with the description, serveto explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof, directed to one of ordinary skill in the art, is setforth in the specification, which makes reference to the appendedfigures, in which:

FIG. 1 is a perspective view of a vehicle equipped with a lift axle.

FIG. 2 is a top schematic view of a truck chassis equipped with a liftaxle.

FIG. 3 is a plot showing the percentage of usage time at various loadsfor 6×4 highway tractor/trailers.

FIG. 4A thru 4D is a flowchart of a method according to a firstembodiment of the present invention covering any general applicationusing one or more principal axles and one or more lift axles.

FIG. 5A thru 5D is a flowchart of a method according to anotherembodiment of the present invention covering a specific application to a6×2 tractor or equivalent that uses only one drive axle and one tagaxle.

FIG. 6 is table of simulated data for an example of the load transferalgorithm for Case 1 where the trailer tires on the lift axle have alower coefficient of rolling resistance than the drive tires found onthe principal or drive axle.

FIG. 7 is a table of simulated data estimating the fuel savings from theCase 1 example shown in the table of FIG. 4.

FIG. 8 is table of simulated data for an example of the load transferalgorithm for Case 2 where the tires on the lift axle have the samecoefficient of rolling resistance as the tires found on the principal ordrive axle.

FIG. 9 is a table of simulated data estimating the fuel savings from theCase 2 example shown in the table of FIG. 6.

FIG. 10 is a graph showing the estimated vehicle fuel savings for Case 1and Case 2 examples. The discontinuity in Case 2 occurs when the tagaxle is lifted off the ground. Note that these cases do not have thesame reference tires and are not applicable to the same situations.

FIG. 11 is a graph showing the estimated vehicle fuel savings for anapplication of invention to trailer axle only (“Case 2 Trailer”) and toboth drive and trailer axle (“Case 1 Drive+Case 2 Trailer”).

The use of identical or similar reference numerals in different figuresdenotes identical or similar features.

DETAILED DESCRIPTION OF THE INVENTION

Since tires are responsible for about 30% of the fuel consumption of atypical long haul tractor-trailer, the inventors recognized that anoptimization of the effective tire rolling resistance of tires of such avehicle would lead to an improvement in the fuel economy of the vehicle.The inventors further recognized that tire rolling resistance isnon-linear, and tires are more efficient (lower coefficient of rollingresistance) when they are subjected to higher loads. In addition, driveaxle tires typically have the highest rolling resistance of any tires onthe vehicle. When, for example, a 6×2 tractor is equipped with a liftaxle, a tractor can transfer load from the tag (non-driven) axle to thedrive axle or vice versa to keep the assembly optimally loaded(typically with the trailer tire at the max legal axle load), therebyincreasing vehicle fuel economy. Provisions are made to ensure that thedrive tire always has sufficient load to ensure adequate traction; theseare detailed below along with an estimation of the tire rollingresistance gains and fuel economy improvements this invention makespossible. It should be noted that this can be accomplished with littlealteration to the existing equipment used on many heavy trucks,tractors, or trailers, which makes it convenient and inexpensive.

The present invention provides a method for improving the vehicle fueleconomy by optimizing the effective tire rolling resistance of the tiresmounted on the vehicle using a lift axle found on the vehicle to adjustthe load on the tires. The method does this by considering the rollingresistance characteristics of the tires and implementing an algorithmbased on those characteristics that raises or lowers the lift axle toadjust the load placed on each of the tires.

The following terms are defined as follows for this disclosure:

“Vehicle” is any type of car, light truck, heavy truck or anything elsethat rolls on a road surface and includes, but is not limited to, atractor, trailer or combination thereof.

“Principal axle” is any axle of a vehicle that is chosen to staystationary with respect to the road surface and may include steer, tag,or drive axles whose load varies significantly with the movement of alift axle.

“Lift axle” is any axle of a vehicle that is capable of and chosen tomove toward or away from the road surface and may include a tag, steer,or drive axle.

“Processing device” includes any form of circuitry, such as amicroprocessor or other microcontroller, or digital signal processor forreceiving data from one or more of sensors and performing certainfunctions therewith as will be further described. In many cases, such adevice will also be fitted with memory. As such, “processing device” asused herein may include one or more microprocessors and may include oneor more memory devices as well.

There are two cases envisioned. In the first, more common case,different tires would be mounted on the lift and the principal axles;for example, in the case where the lift axle is tag axle, it would bemounted with trailer tires and in the case where the principal axle is adrive axle, it would be mounted with drive tires. Trailer tirestypically have a lower coefficient of rolling resistance than drivetires. This situation is designated as Case 1.

In the second case, the tires on the lift axle have a coefficient ofrolling resistance that is equal to or greater than the coefficient ofrolling resistance of the tires mounted on the principal axle. This mayoccur, for example, when the same tires are mounted on both drive andlift axles of a 6×2 tractor. This situation is designated as Case 2.Efficiencies may still be made in this case due to the non-linear natureof tire rolling resistance, which causes two tires operating at halfload to be less efficient than one tire operating at full load, forexample. This is due to the power law relationship between tire load androlling resistance. A commonly used description known in the art of theevolution of the rolling resistance coefficient of a tire CIT with loadis given below by Equation 1:

$\begin{matrix}{{{C_{rr}(z)} = {{C_{rr}\left( z_{ref} \right)}\left( \frac{z}{z_{ref}} \right)^{- 0.1}}};} & {{Eq}.\mspace{14mu} 1}\end{matrix}$

where z is the load and z_(ref) is the reference load at which therolling resistance coefficient was measured.

In the following paragraphs the algorithms of the present invention areoutlined for distributing the load between the lift and principal axlesin order to optimize the fuel economy of the vehicle. The presentinvention involves the load transfer for fuel economy; this isaccomplished by loading the tire which is operating most efficiently asmuch as possible, i.e. increase the load on the tires having the lowestcoefficient of rolling resistance. A method for using these algorithmsis represented by the flow chart contained in FIGS. 4A-4D and 5A-5D. Itcomprises a first step 100, 200 that includes determining whether a Case1 or Case 2 scenario is present, that is to say, whether the rollingresistance characteristics of the tires mounted on a lift axle are lowerthan those on a principal axle for the general case application, and forthe case of a 6×2 tractor or equivalent application, whether the rollingresistance characteristics for the tires mounted on the tag axle arelower than those on a drive axle.

It should be noted that these algorithms are based on the presumptionthat the load taken by the front steer axle is largely fixed anddependent on the configuration of the vehicle, therefore its effect wasconsidered to be negligible. Therefore, reference to the “total vehicleload” herein including the claims with respect to the algorithms doesnot include the load carried by the front steer axle of a vehicle whensaid steer axle load does not vary significantly with movement of a liftaxle. However, “total vehicle load” as used herein including the claimsdoes include the load carried by the front steer axle of a vehicle whensaid steer axle load does vary significantly with movement of the liftaxle.

For Case 1, the algorithm unloads the principal axle, keeping the liftaxle as fully loaded as possible and allowing it to operate moreefficiently. When the principal axle load has reached the thresholddesired for maintaining traction on the principal axle, which is oftenthe drive axle, the lift axle load is then reduced. As the load reducesstill further, if and when the lift axle, which is often the tag axle,reaches a threshold load, the load is then shared evenly between bothaxles. The algorithm for determining the load on the lift and theprincipal axle is as follows:

If Ltotal>(Lmax)*Nlift+(Lmin)*Nprincipal then the lift axle(s) is (are)moved until Llift=Lmax and Lprincipal=[Ltotal−(Lmax*Nlift)]/Nprincipal(this generally relates to steps 110 and 120 of the flowcharts found inFIGS. 4A, 4B and 4D),If Ltotal<=(Lmax)*Nlift+(Lmin)*Nprincipal but is greater than Ntot*Lminthen the lift axle(s) is (are) moved untilLlift=[Ltotal−Lmin*Nprincipal]/Nlift and Lprincipal=Lmin (this generallyrelates to steps 130 and 140 of the flowcharts found in FIGS. 4A, 4B and4D),If Ltotal<=Ntot*Lmin then the lift axle(s) is (are) moved untilLprincipal=Llift=Ltotal/Ntot (this generally relates to steps 150 and160 of the flowcharts found in FIGS. 4A, 4B and 4D),where Lmax=Maximum allowable axle load, Lmin=Minimum allowable principalaxle load required for traction, cornering, etc., Ltotal=Measured totalload on all axles, Llift=Desired load on the lift axle,Lprincipal=Desired load on the principal axle, Nlift=the number of liftaxles on the vehicle, Nprincipal=the number of principal axles on thevehicle, and Ntot is total number of principal and lift axles addedtogether.

Of course, most scenarios involve the use of a 6×2 tractor, orequivalent, that has only one drive axle as the principal axle and onetag axle as the lift axle. In such an application, Nlift=1, Nprincipal=1and Ntot=2. Then the above algorithm simplifies to the following:

Ltotal>Lmax+Lmin then the tag axle is moved until Ltag=Lmax andLdrive=Ltotal−Lmax (this generally relates to steps 210 and 220 of theflowcharts found in FIGS. 5A, 5B and 5D),

If Ltotal<=Lmax+Lmin but is greater than 2*Lmin then the tag axle ismoved until Ltag=Ltotal−Lmin and Ldrive=Lmin (this generally relates tosteps 230 and 240 of the 250 flowcharts found in FIGS. 5A, 5B and 5D),If Ltotal<=2*Lmin and Ltotal>=Lmin then the tag axle is moved untilLdrive=Ltag=Ltotal/2 (this generally relates to steps 250 and 260 of theflowcharts found in FIGS. 5A, 5B and 5D),where Lmax=Maximum allowable axle load, Lmin=Minimum allowable driveaxle load required for traction, cornering, etc., Ltotal=Measured totalload on all axles, Ltag=Desired load on the tag axle and Ldrive=Desiredload on the drive axle. Moving the tag axle until the drive and tagaxles have the same load is preferable at very low vehicle loads ascompared to lifting off the lift axle entirely since the penalty ofincreasing the load on the drive tires, which have higher rollingresistance coefficients than the tires on the tag axle, is greater thanthe benefit of improving efficiency by increasing the load on the drivetires.

This algorithm is illustrated in FIG. 6 for use with a 6×2 tractor andassociated trailer. For purposes of example the drive and trailerrolling resistance values (8.2 Kg/ton and 6.0 Kg/ton respectively) weretaken from the baseline scenario from the NTHSA/EPA proposed fuelefficiency standards (see Draft Regulatory Impact Analysis, “ProposedRulemaking to Establish Greenhouse Gas Emissions Standards and FuelEfficiency Standards for Medium- and Heavy-Duty Engines and Vehicles”,EPA-420-D-10-901, October 2010.). The maximum axle load Lmax was takento be 17,000 lbs, corresponding to the typical maximum legal axle load.Similarly, the minimum axle load was taken to be 3500 lbs correspondingto a typical empty trailer load. Total load range is twice the minimumaxle load to twice the maximum axle load. Other values of Lmax, Lmin andtire rolling resistance can be selected as appropriate to conditions.

The reduction in tire rolling resistance as a function of Ltotal isshown for Case and compared with the usual vehicle configuration inwhich there is no load transfer between the axles. At each total loadLtot, the algorithm determines the load Ldrive and Ltag for the twoaxles. The tire rolling resistance coefficient is then corrected for theactual tire load using Equation 1. The effective rolling resistance forthe combined assembly is calculated by multiplying each axle load by itscorrected tire rolling resistance coefficient to find the rollingresistance force. These forces are added for the two axles and thendivided by the total load to obtain the effective rolling resistancecoefficient for the assembly. The difference in the effective rollingresistance coefficient is the displayed in absolute terms and as apercentage.

In FIG. 7, an estimate of the vehicle fuel savings that Case 1 offers isprovided. The methodology for this estimate is to determine totalvehicle rolling resistance by calculating the rolling resistance forcesfor each axle and dividing by the total load of the vehicle. Thiscalculation used the baseline rolling resistance coefficient from theNTHSA/EPA proposed fuel efficiency standards for the steer tire (7.8Kg/ton). A constant load Lsteer of 12,000 lbs on the steer axle isassumed as well as an equal load on the drive and trailer assemblies.The effective rolling resistance coefficient of the entire vehicle isreported as a function of Ltotal for both Case 1 and for the usual caseof no load transfer. Estimated fuel savings are taken to be 30% of thispercentage difference (this assumption is contained in Barand, J.,Bokar, J., “Reducing Tire Rolling Resistance to Save Fuel and LowerEmissions”, presented at SAE World Congress and Exhibition, SAE2008-01-0154, Detroit. 2008.), based upon the typical contribution oftires to overall vehicle fuel consumption.

Turning now to Case 2, an algorithm is provided that unloads the liftaxle, keeping the principal axle as fully loaded as possible andallowing it to operate more efficiently as indicated by Equation 1. Whenthe lift axle load drops below a certain threshold such as the minimumload allowed for an axle, the principal axle is incrementally decreaseduntil the total load reaches the maximum allowable load per axle, atwhich time the lift axle is lifted completely off of the ground and theentire load is supported by the principal axle alone. This case has theadvantage of improving traction at all speeds and improving the wearprofile. The algorithm for determining the load on the lift and theprincipal axles is as follows for a case where the number of lift axlesis equal to or less than the number of principal axles:

Using the definitions for the following variables:Lmax=Maximum axle loadLmin=Minimum axle load for traction, cornering, etc.Ltotal=Measured total load on all axlesLlift=Desired load on lift axleLprincipal=Desked load on principal axleNprincipal=Number of principal axlesNlift=Number of lift axles

Ntot=Nprincipal+Nlift

The load is defined by the following procedure:If Ltotal−(Lmax)*Nprincipal>Lmin*Nlift, then the lift axle(s) is (are)moved untilLprincipal=Lmax and Llift=[Ltotal−(Lmax*Nprincipal)]/Nlift (thisgenerally relates to steps 170 and 180 of the flowcharts found in FIGS.4A, 4C and 4D);If Ltotal−Lmax*Nprincipal<=Lmin*Nlift, and if Ltotal>=Ntot*Lmin, thenthe lift axle(s) is (are) moved untilLprincipal=Ltotal−(Lmin*Nlift)/Nprincipal and Llift=Lmin (this generallyrelates to steps 190 and 192 of the flowcharts found in FIGS. 4A, 4C and4D);If Ltotal<Ntot*Lmin, then Lprincipal=Ltotal/Nprincipal and Llift=0; thatis to say that the lift axles are lifted from the ground (this generallyrelates to steps 195 and 197 of the flowcharts found in FIGS. 4A, 4C and4D).

For rare cases where the number of principal axles is less than thenumber of lift axles and the rolling resistance characteristics of thetires mounted on the lift axles is the same as those of the tiresmounted on the principal axles, then the goal is to load the lift axlesas much as possible and the highest loading regime and the associatedmovement step for Case 2 would be similar to steps 110 and 120 for aCase 1 scenario above, and the intermediate loading regime andassociated movement step for a Case 2 scenario that correspond to steps190 and 192 above, is as follows:

If Ltotal−Lmax*Nlift<=Lmin*Nprincipal and if Ltotal>=Ntot*Lmin, then thelift axle(s) is (are) moved until Llift=Lmax andLprincipal=[Ltotal−(Lmax*Nlift)]/Nprincipal (these steps are not shownin flowcharts).

For the same rare case where the number of principal axles is less thanthe number of lift axles, then the bottom loading regime and associatedmovement step for a Case 2 scenario that correspond to steps 195 and 197above, is as follows:

If Ltotal<Ntot*Lmin, then Llift=Ltotal/Nlift and Lprincipal=0, that isto say that a lift axle is moved until there is no load on a principalaxle (these steps are not shown in the flowcharts).

Again, most scenarios involve the use of a 6×2 tractor, or equivalent,that has only one drive axle as the principal axle and one tag axle asthe lift axle. In such an application, Nlift=1, Nprincipal=1 and Ntot=2.Then the above algorithm that covers the scenario where the number oflift axles is less than or equal to the number of lift axles simplifiesto the following using the definitions for the following variables:

Lmax=Maximum axle loadLmin=Minimum tag axle loadLtotal=Measured total load on tag+drive axleLtag=Desired load on tag axleLdrive=Desired load on drive axleThe load is defined by the following procedure:If Ltotal−Lmax>Lmin, then the tag axle is moved until Ldrive=Lmax andLtag=Ltotal−Lmax (this generally relates to steps 270 and 280 of theflowcharts found in FIGS. 5A, 5C and 5D);If Ltotal−Lmax<=Lmin, and if Ltotal−Lmax>0, then the tag axle is moveduntil Ldrive=Ltotal−Lmin and Ltag=Lmin (this generally relates to steps290 and 292 of the flowcharts found in FIGS. 5A, 5C and SD and keepingthe load on the tag axle at Lmin helps prevent undesirable wearprofiles);If Ltotal−Lmax<=0, Ldrive=Ltotal and Ltag=0; that is to say the tag axleis lifted off the ground (this generally relates to steps 295 and 297 ofthe flowcharts found in FIGS. 5A, 5C and 5D).

This algorithm is illustrated in FIG. 8 for a 6×2 tractor application.In keeping with the scenario, the drive and tag rolling resistancevalues were taken to both be the baseline drive tire from the NTHSA/EPAproposed fuel efficiency standards (8.2 Kg/ton) (see Draft RegulatoryImpact Analysis, “Proposed Rulemaking to Establish Greenhouse GasEmissions Standards and Fuel Efficiency Standards for Medium- andHeavy-Duty Engines and Vehicles”, EPA-420-D-10-901, October 2010.). Themax axle load Lmax was taken to be 17,000 lbs, corresponding to typicalmaximum legal axle load. Similarly, the minimum axle load was taken tobe 3500 lbs corresponding to a typical empty trailer load, Total loadrange is twice the minimum axle load to twice the maximum axle load.Other values of Lmax, Lmin and tire rolling resistance can be selectedas appropriate to conditions.

As in Case 1, the reduction in tire rolling resistance as a function ofLtotal is shown for Case 2 and compared with the usual vehicleconfiguration in which there is no load transfer between the axles. Ateach total load Ltot, the algorithm determines the load Ldrive and Ltagfor the two axles. Note that in Case 2 the tag axle is lifted off theground at a total axle load Ltot=17,000 lbs. The tire rolling resistancecoefficient is then corrected for the actual tire load using equation 1.The effective rolling resistance for the combined assembly is calculatedby multiplying each axle load by its corrected tire rolling resistancecoefficient to find the rolling resistance force. These forces are addedfor the two axles and then divided by the total load to obtain theeffective rolling resistance coefficient for the assembly. Thedifference in the effective rolling resistance coefficient is thedisplayed in absolute terms and as a percentage.

In FIG. 9, an estimate of the vehicle fuel savings that Case 2 offers isgiven. The methodology and description of the columns are the same asfor Table 2 above.

FIG. 10 displays the estimated vehicle fuel savings from Case 1 and Case2. They vary significantly with vehicle load. In Case 1 the tag axle isa more efficient tire and the load is transferred to this tireprogressively. Maximum fuel savings occur when the drive axle has beenunloaded as much as permitted and the tag axle is loaded as much aspermitted. In Case 2 the load is transferred to the drive tire forsimplicity, added traction and improved wear profile of the drive tire.When the rolling resistance of the tag axle is equal to or greater thanthe drive tire, this is the more efficient scenario. A discontinuity inCase 2 occurs when the tag axle is lifted off the ground, at which pointthe maximum fuel economy savings are achieved.

It is contemplated that this invention is equally applicable to thevehicle trailer axles, provided they are equipped with a suitablemechanical system and/or electronic control system to accomplish theload transfer. For example, both axles would be equipped with identicaltrailer tires, as is the usual practice, so that Case 2 would apply.Using the 6.0 Kg/ton rolling resistance coefficient from the baselinescenario from the NTHSA/EPA proposed fuel efficiency standards fortrailer tires, we can apply the identical calculations detailed in Table3 and Table 4 to obtain a Case 2 estimate for the trailer axle. This isshown as “Case 2 Trailer” on FIG. 11 along with the data previouslypresented in FIG. 10, for comparison. It can be seen that the gains aresomewhat less than with the drive tires used in Case 2, clue to thegreater efficiency (lower Cri) of the trailer tires.

Finally, the estimate for trailer axles can be combined with the Case Iscenario for the drive axle (the most likely scenario in practice) toestimate the gains for applying this invention to both tractor andtrailer axles simultaneously. As can be seen, gains of between 1% and 2%are expected over a large section of the operating load of a typicalheavy truck, which is a significant amount of savings for those whoroutinely operate such vehicles.

These algorithms can be used by an operator, mechanic or other workerassociated with vehicles of any type, including heavy trucks eithermanually or automatically by entering data into a program that isexecuted by a processing device such as an electronic control systemsuch as that disclosed by U.S. Pat. No. 7,222,867, the contents of whichare incorporated herein by reference in its entirety.

A vehicle electronic control system is a generalization of applicationsof contemporary digital networks to motor vehicles, and mayadvantageously be based on the Society of Automotive Engineers SAE J1939standard for controller area networks. An SAE J1939 compliant businterconnects a plurality of controllers provided for primary vehiclefunctions. Among these controllers are an engine controller, atransmission controller (for automatic and semi-automatic equippedvehicles), an electronic control system controller (ESC) and,potentially, a stability and height (suspension) controller. The ESC mayalso connect with a SAE J1708 bus over which it communicates with agroup of switches which in turn include control switches for positioninga lift axle. The ESC can contain program instructions in its memory forautomatic control of the lift axle. The instructions generated by theESC may be coded as J1939 messages that are broadcast over the bus, andare then decoded and carried out by a solenoid controller that isconnected to the bus. Lastly, the solenoid controller generates theactual control signals applied to the solenoids that affect movement ofthe lift axle.

An exemplary embodiment of a method of the present invention may beimplemented in the following manner. First, the rolling resistancecharacteristics of the tires mounted on the lift and principal axles, asconsistent with steps 100 and 200 of the flowcharts, are analyzed to seewhether a case 1 or case 2 scenario is present. For example, an operatormay enter the rolling resistance characteristics of the tires mounted onthe principal and lift axles of the vehicle via an input device such asa keyboard, touchscreen, mouse, etc. Also, the number and type of axleson the vehicle may be entered. Alternatively, the tires mounted on theaxles of the vehicle may have RFID chips that transmit to an inputdevice such as a receiver the rolling resistance characteristics of eachtire which can be stored in memory and the type of axles and numberthereof could also be preprogrammed. If this method is being performedmanually, then the operator takes a mental note of the rollingresistance characteristics of these tires and the number and types ofaxles. These and other means known in the art or that will be devised inthe art could be used to accomplish steps 100 and 200 of the presentinvention.

Given this data, the processing device such as an electronic controlsystem or the operator then determines if any sets of tires mounted onthe various axles fall into Case 1 or Case 2 categories. If so, then theappropriate algorithms that are stored in memory can be executed by theprocessing device and can be applied to the appropriate sets of tiresdepending on their rolling resistance characteristics during the use ofthe vehicle automatically. Additionally, the system could alert theoperator using a telltale sign such as some visual or audio cue-sent byan output device that the lift axle should be moved, in which case, theoperator can initiate movement of the lift axle by activating a switch.The movement of the lift axle can be based on measurements that aretaken continually, periodically, averaged over time or by other meansknown in the art. When performed manually, readings may be taken by aperson such as an operator each time an action is performed where theoperator believes a material change in the weight of the vehicle hasbeen affected, such as when a loading or unloading operation has beendone. Once the need to move the lift axle has been identified, the liftaxle is moved until the suitable loads are applied to the various axlesof the vehicle. Steps 110 thru 197 as well as steps 210 thru 297 may beimplemented using any of these means just described or by other meansknown in the art or that will be developed in the art that have the samefunction.

In many cases, the algorithms will be used where the weight that thevehicle is carrying is decreased over time. It is possible that thealgorithms can be used for applications where the weight that thevehicle is carrying is increased over time. Monitoring of the vehicleload can be clone by in any manner known in the art or as describedpreviously.

When an automated system is provided in a vehicle in accordance with thepresent invention, it may be provided by the OEM with the algorithmsdescribed herein already programmed into a processing device that ispart of an electronic control system. Otherwise, vehicles withelectronic control systems may be retrofitted with these algorithms bydownloading programs containing the algorithms using some sort ofcomputer readable medium, by downloading them using satellite or otherwireless technology, or by other means commonly known in the art.

While the present subject matter has been described in detail withrespect to specific exemplary embodiments and methods thereof, it willbe appreciated that those skilled in the art, upon attaining anunderstanding. of the foregoing may readily produce alterations to,variations of, and equivalents to such embodiments. For example, thesteps contained in the method described herein regarding determiningwhat category the total load of the vehicle falls into may be done inmay be executed in any order as long as the appropriate category iseventually found. Accordingly, the scope of the present disclosure is byway of example rather than by way of limitation, and the subjectdisclosure does not preclude inclusion of such modifications, variationsand/or additions to the present subject matter as would be readilyapparent to one of ordinary skill in the art.

1. A method for improving the fuel economy of a vehicle by optimizingeffective tire rolling resistance, the method comprising the followingsteps: providing a vehicle with at least one principal axle and one liftaxle, said axles having tires mounted thereon; and determining whetherthe total load carried by the vehicle is greater than the sum of maximumallowable axle load multiplied by the number of lift axles added to theminimum allowable axle load necessary multiplied by the number ofprincipal axles; or determining whether the total load carried by thevehicle is less than or equal to the sum of maximum allowable axle loadmultiplied by the number of lift axles added to the minimum allowableaxle load necessary multiplied by the number of principal axles whilealso being greater than the total number of axles multiplied by theminimum allowable axle load necessary, or determining whether the totalload carried by the vehicle is less than or equal to the total number ofaxles multiplied by minimum allowable axle load necessary; and furthercomprising the step of determining that the tires on a lift axle havelower rolling resistance characteristics than the tires on a principalaxle.
 2. The method of claim 0, wherein the total load carried by thevehicle is determined to be greater than the sum of the maximumallowable axle load multiplied by the number of lift axles added to theminimum allowable axle load necessary multiplied by the number ofprincipal axles, said method further comprising the step of moving atleast one lift axle so that a lift axle carries the maximum allowableaxle load and a principal axle carries less load than one lift axle. 3.The method of claim 0, wherein the total load carried by the vehicle isdetermined to be less than or equal to the sum of maximum allowable axleload multiplied by the number of lift axles added to the minimumallowable axle load necessary multiplied by the number of principalaxles while also being greater than the total number of axles multipliedby the minimum allowable axle load necessary, said method furthercomprising the step of moving at least one lift axle so a principal axlecarries a minimum axle load and a lift axle carries a load greater thana principal axle.
 4. The method of claim 0, wherein the total loadcarried by the vehicle is determined to be less than or equal to thetotal number of axles multiplied by the minimum allowable axle loadnecessary, said method further comprising the step of moving at leastone lift axle so a principal axles carries the same load as a lift axle.5. The method of any of the preceding claims in which the total numberof axles on the vehicle is two, wherein said at least one principal axleis a drive axle and said at least one lift axle is a tag axle, andwherein said determining steps include: determining whether the totalload carried by the vehicle is greater than the sum of the maximumallowable axle load and minimum allowable axle load; or determiningwhether the total load carried by the vehicle is less than or equal tothe sum of maximum allowable axle load and minimum allowable axle loadwhile also being greater than two multiplied by the minimum allowableaxle load; or determining whether the total load carried by the vehicleis less than or equal to two multiplied by the minimum allowable axleload while also being greater than or equal to the minimum allowableaxle load.
 6. The method of claim 5, wherein the total load carried bythe vehicle is determined to be greater than the sum of maximumallowable axle load and the minimum allowable axle load, said methodfurther comprising the step of moving the tag axle until the axle loadof the tag axle is the maximum allowable axle load and the axle load ofthe drive axle is equal to total load carried by the vehicle minus themaximum allowable axle load.
 7. The method of claim 5, wherein the totalload carried by the vehicle is determined to be less than or equal tothe sum of maximum allowable axle load and minimum allowable axle loadwhile also being greater than two multiplied by the minimum allowableaxle load, said method further comprising the step of moving the tagaxle until the tag axle load equals the total load carried by thevehicle minus the minimum axle load and drive axle load equals theminimum allowable axle load.
 8. The method of claim 5, wherein totalload carried by the vehicle is determined to be less than or equal totwo multiplied by the minimum allowable axle load while also beinggreater than or equal to the minimum allowable axle load, said methodfurther comprising the step of moving the tag axle until drive axle loadequals the tag axle load.
 9. (canceled)
 10. A method for improving thefuel economy of a vehicle by optimizing effective tire rollingresistance, the method comprising the following steps: providing avehicle with at least one principal axle and one lift axle, said axleshaving tires mounted thereon; and determining whether the quantity ofthe total load carried by the vehicle minus the product of the maximumallowable axle load multiplied by the number of principal axles isgreater than the minimum allowable axle load necessary multiplied by thenumber of lift axles; or determining whether the quantity of the totalload carried by the vehicle minus the product of the maximum allowableaxle load multiplied by the number of principal axles is less than orequal to the product of minimum allowable axle load multiplied by thenumber of lift axles while at the same time the total load is greaterthan or equal to the total number of axles multiplied by the minimumallowable axle load necessary, or determining whether the quantity ofthe total load carried by the vehicle is less than the total number ofaxles multiplied by the minimum allowable axle load necessary, ordetermining whether the quantity of the load carried by the vehicleminus the product of the maximum allowable axle load multiplied by thenumber of lift axles is less than or equal to the product of the minimumallowable axle load multiplied by the number of principal axles while atthe same time the total load is greater than or equal to the totalnumber of axles multiplied by the minimum allowable axle load necessary;and further comprising the step of determining that the tires on a liftaxle have the same or higher rolling resistance characteristics than thetires on a principal axle.
 11. The method of claim 10, wherein thequantity of the total load carried by the vehicle minus the product ofthe maximum allowable axle load multiplied by the number of principalaxles is determined to be greater than the minimum allowable axle loadnecessary multiplied by the number of lift axles, said method furthercomprising the step of moving at least one lift axle so a principal axlecarries the maximum allowable axle load.
 12. The method of claim 10,wherein the quantity of the total load carried by the vehicle minus theproduct of the maximum allowable axle load multiplied by the number ofprincipal axles is determined to be less than or equal to the minimumallowable axle load necessary multiplied by the number of lift axleswhile at the same time the total load carried by the vehicle is greaterthan or equal to the total number of axles multiplied by the minimumallowable axle load necessary, said method further comprising the stepof moving at least one lift axle so a lift axle carries the minimumallowable axle load, or wherein the quantity of the total load carriedby the vehicle minus the product of the maximum allowable axle loadmultiplied by the number of lift axles is determined to be less than orequal to the minimum allowable axle load necessary multiplied by thenumber of principal axles while at the same time the total load carriedby the vehicle is greater than or equal to the total number of axlesmultiplied by the minimum allowable axle load necessary, said methodfurther comprising the step of moving at least one lift axle so aprincipal axle carries the minimum allowable axle load.
 13. The methodof claim 10, wherein the quantity of the total load carried by thevehicle is determined to be less than the total number of axlesmultiplied by the minimum allowable axle load necessary, said methodfurther comprising the step of moving at least one lift axle so a liftaxle carries no load or so that a principal axle carries no load. 14.The method of any one of claims 10 to 13, in which the total number ofaxles on the vehicle is two, wherein said at least one principal axle isa drive axle and said at least one lift axle is a tag axle, and whereinsaid determining steps include: determining whether the total loadcarried by the vehicle minus the maximum allowable axle load is greaterthan the minimum allowable axle load; or determining whether the totalload carried by the vehicle minus the maximum allowable axle load isless than or equal to the minimum allowable axle load while also beinggreater than zero; or determining whether the total load carried by thevehicle minus the maximum allowable axle load is less than or equal tozero.
 15. The method of claim 14, wherein the quantity of the total loadcarried by the vehicle minus the maximum allowable axle load isdetermined to be greater than the minimum allowable axle load, saidmethod further comprising the step of moving the tag axle so the driveaxle carries the maximum allowable axle load.
 16. The method of claim14, wherein the quantity of the total load carried by the vehicle minusthe maximum allowable axle load is determined to be less than or equalto the minimum allowable axle load while at the same time the total loadcarried by the vehicle is greater than the maximum allowable axle loadnecessary, said method further comprising the step of moving the tagaxle so that the tag axle carries the minimum allowable axle load. 17.The method of claim 14, wherein the quantity of the total load carriedby the vehicle minus the maximum allowable axle load is determined to beless than or equal to zero, said method further comprising the step ofmoving the tag axle so the tag axle carries no load.
 18. (canceled) 19.A system for improving vehicle fuel economy for a vehicle having atleast one lift axle and one principal axle, comprising: tires mounted onthe lift and principal axles; an input device for entering instructionsor information; a memory for storing instructions, data or programshaving algorithms; load sensors for determining the loads on theprincipal and lift axles; a lift mechanism for lowering or raising thelift axle; at least one processing device in communication with saidinput device, said memory, said load sensors, and said lift mechanism,said processing device configured for receiving measurements indicativeof the load placed on the principal and lift axles as well as the totalload carried by the vehicle; executing an algorithm depending on whetherthe tires on the lift axle have rolling resistance characteristics thatare higher, lower or the same as the tires on the principal axle andsubsequently sending a signal to the lift mechanism in order to move thelift axle to change the loads placed on the principal and lift axles;stopping the movement of the lift mechanism once the desired loads onthe principal and lift axles have been reached; and, monitoring thetotal vehicle load over time.
 20. The system of claim 19, furthercomprising an output device for alerting the operator that the liftmechanism should be activated to adjust the load on the principal andlift axles so that the operator can signal the processing device toactivate the lift mechanism.
 21. The system of claim 19, furthercomprising an RFID chip attached to the tire, said RFID configured forstoring and sending information concerning the rolling resistancecharacteristics of the tire to the processing device via the inputdevice.
 22. The system of claim 19, further comprising a SAE J1939compliant bus that interfaces with an electronic control system thatacts as the processing device for controlling the lift mechanism. 23.The system of claim 22, further comprising a SAE J1708 compliant busthat is connected with said electronic control system, said SAE J1708compliant bus also communicating with switches for positioning the liftaxle.